Method of treating dry plate elements



METHOD OF TREATING DRY PLATE ELEMENTS Filed June 5, 1939 T0 SOURCEDEADJ'USTABLY VAR/ABLE D/RECT VOLTAGE.

T0 SOURCE OF ALTER/VAT/NG' VDLTAGE HAW/VG S/NE WAVE-FDR.

COUNTER ELECTRODE.

BLOCK/N6 LAYER.

SEHI-CDNDUCUNG LAYER. CARRIER ELECTRODE.

T0 SOURCE DFADJUSTABLY VAR/ABLE DIRECT VOLTAGE BLOCKING LAYER AL 7' ERNA 77N6 VOLTAGE SEMI CONDUCT/N6 LAYER HAVING S/IVE WAVE -FORM. CARR/ERELECTRODE T 0 SOURCE OF BIAS/N6 DIRECT VOLTAGE.

Inventor:

Heinz Wittke His Attorney.

atented Apr. 8, 1941 METHOD OF TREA ELEME mo. nay rLsrE NTS HeinzWittke, Hoheu-Neuendorf, near Berlin,

Germany,

assignor to General Electric Company, a corporation of New'YorkApplication June 5, 1939, Serial No. 277,503 In Germany June 14, 1938 4Claims.

My invention relates to electrode systems of non-symmetricalconductivity such as dry plate cells for rectifiers or the like, andparticularly to methods of treating dry plate elements subsequent totheir manufacture for the purpose of artificial aging of the elements,or for obviating the danger of short circuits. One of its objects is toprovide improved methods of such subsequent electrical treatment.

lit is well known that electrode systems oi nonfore upon testing suchdry plate elements with service current they become short circuitedanddamaged, and are consequently rejected.

Further, in case the dry plate element is loaded with alternatingcurrent but without limiting resistance in the circuit, then, since thearc origihating at the short circuit points is extinguished symmetricalconductivity such as dry plate elements, for example selenium rectifiercells comprising a carrier electrode, a semi-conducting layer, ablocking layer, and a counter-electrode, require after their productionby chemical means and by heat treatment a subsequent electricaltreatment which accomplishes the artificial aging of the element andremoves the danger of damage in service from internal short circuitswhich are either already present in the element or which would originatein service by reason of the sparking over of the current at the weakestpoints of the blocking layer. The heat produced upon the passage of thecurrent during the above mentioned subsequent electrical treatment burnsout the points subject to short circuit. Prior methods have employed forthis purpose either direct current or alternating current with orwithout limiting resistance in the circuit.

These prior methods have been found to entail considerabledisadvantages. In case the dry plate element is loaded with directcurrent without limiting resistance, the danger exists that if the shortcircuit which occurs when the current is applied is a severe one theheat developed thereat becomes so great that the burning out of theshort circuit area takes place with sufficient violence to fuse thecounter-electrode in the region spreading out from the short circuitarea. The fusion also penetrates deeply into the semi-conductor layerthereby causing the short circuit to become still more violent, the dryplate element being finally destroyed. In selenium rectiflers, for example, under such circumstances externally visible craters appear havingslightly raised edges produced by the molten metal. a

On the other hand in case the dry plate element is loaded with directcurrent or alternating current through a limiting resistance the dangerexists that certain points or areas or a given dry plate element may beso defective as tobe liable to short circuit in service but that, byreason of the limiting resistance, a current intensity suflicient toburn out these defective points or incipient short circuits cannotdevelop. Thereduring each half period of the alternating current, damageto the plates of the element is avoided during these hali periods.However, in the halt periods corresponding to the current conducting orflow direction of the element a current originates which is so intensethat owing to the heat developed by this current the counter electrodefuses entirely or in part and thereby the element is finally destroyed.In selenium rectlfiers under these circumstances. externally visibleblisters occur in the counter electrode.

In accordance with the present invention the foregoing disadvantagesare. overcome, the meth-- od oi subsequent treatment of dry plateelements after production being carried out in such manner that thesalvaging of elements having defective points or areas tending to causeshort circuit in service is successfully accomplished without danger ofdestruction of the elements during the salvaging operation.

Specifically, in accordance with the present invention the dryplateelements are treated, subsequent to production, in such manner thatan alternating voltage isapplied thereto the maximum instantaneousvalues of which are 1 greater in the blocking direction of the elementsiii than in their current conducting or flow direction.

The novel features which are considered to be characteristic of myinvention are set forth with particularity in the appended claims. Myinvention itself, however, both as to its organization and method ofoperation together with further objects and advantages thereof may bestbe understood by reference to the following description taken inconnection with the accompanying drawing wherein Fig. 1 is a circuitdiagram illustrating a method whereby my invention may be practiced, andFig. 2 is a circuit diagram illustrating a modification of the methodillustrated in Fig. 1.

Referring to Fig. 1, the dry plate element I which is to be electricallytreated subsequently to being formed is connected, as through atransformer 2, to a source of alternating voltage having a sinewave-form. A suitable direct voltage is superimposed on the alternatingvoltage, for

" example by connecting in series in the transformer. and dry plateelement circuit 3 a source of adjustably variable direct voltage. Thepolarity of the latter source is so arranged that the instantaneousvalues of the resultant voltage in circuit 3 impressed on element I aresubstantially greater in the blocking direction of the element, in thepresent case from carrier electrode 4 to counter-electrode 5, than inthe current conducting or flow direction of the element.

In the circuit illustrated in Fig. 2 instead of impressing analternating voltage of pure sine wave form on the element I, analternating voltage deviating from this form is employed which isimpressed on the element i through a transformer 6. Such distortedalternating voltages may be'generated, for example, as shown in Fig. 2by applying in a usual manner a direct current bias to the transformer Bfrom a suitable source of direct voltage. The polarity of the directbiasing voltage source is so arranged that from the resulting distortedalternating voltage impressed on the element i, a substantially highereffective voltage is impressed thereon in the blocking direction than inthe current conducting or flow direction. Further, in the circuit ofFig. 2 employing an alternating voltage deviating from the sinewave-form, it is advantageous to provide in series with this alternatingvoltage impressed on the element I, an appropriately variable directvoltage as in Fig. 1, in order, for example, to control the time periodsof the positive and negative portions or voltage pulses of the distortedvoltage waves at a given ratio of the maximum values of these positiveand negative portions. In this manner the current, and the heat producedthereby in the element, during the flow direction periods thereof remainwithin moderate limits, whereas during the blocking direction periodssuch a high voltage is applied to the element that at any defectivepoints in the blocking layer, arcs are caused to occur. Thus the shortcircuits originating by reason of the high applied voltage, as well asshort circuits already existing in the element, are burned out by theheating current due to the high voltage.

The arcs cannot produce the disturbing results referred to hereinbeforein connection with the employment of direct current, since in the use ofthe present method, the arc is extinguished at the end of each halfperiod in the blocking direction. In the use of the present methoddestruction of the element by fusion of the counter-electrode, whichoccurs when alternating current is employed without limiting resistance,is likewise prevented since, for the flow direction in the element, theinstantaneous values of the alternating voltage are selected so smallthat they remain below the voltage values occurring, in the flowdirection, in actual service.

Nothing in the present method prevents the use therewith of limitingresistances, but the use of such resistances is not necessary since thevalues of the instantaneous voltages applied to the element may be soselected that the desired current intensities are obtained in both theblocking direction and in the flow direction.

The method in accordance with the present invention is of importanceprimarily in connection with dry plate rectifiers, but it is alsoapplicable in connection with blocking layer photo cells or resistancecells. Further, the method may be utilized in connection with severaldry plate systems connected in series or in parallel.

My invention has been described herein in particular embodiments forpurposes of illustration. It is to be understood, however, that theinvention is susceptible of various changes and modifications and thatby the appended claims I intend to cover any such modifications as fallwithin the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:'

1. The method of treating a dry plate element of non-symmetricalconductivity which includes subjecting said element to an alternatingvoltage the maximum instantaneous values of which in the blockingdirection of said element are sufficiently high to burn outshort-circuit areas of said element and in the current conductingdirection of said element are substantially lower than the maximuminstantaneous values of the voltage impressed upon said element in thecurrent conducting direction in normal use of said element forrectification.

2. The method of treating a dry plate element of non-symmetricalconductivity which includes providing an alternating voltage having asine wave-form, impressing a direct voltage upon said alternatingvoltage to produce a resultant voltage, and impressing said resultantvoltage upon said element, the peak value of said alternating voltageand the value of said direct voltage being each relatively high to causethe peak value of the resultant sum voltage in the blocking direction tobe sumciently high to burn out shortcircuit areas of said element andsaid peak value of the alternating voltage exceeding the value of thedirect voltage by only a relatively small amount to cause the peak valueof the resultant difference voltage in the current conducting directionto be substantially less than the peak value of alternating voltageimpressed upon said element in, the current conducting direction innormal use of said element for rectification.

-3. The method of treating a dry plate element of non-symmetricalconductivity which includes impressing a distorted alternating voltageupon said element in such manner that the maximum instantaneous valuesof the voltage pulses of said resultant voltage are substantiallygreater in the blocking direction of said element than in the currentconducting direction thereof, and impressing a direct voltage upon saidresultant voltage to determine the time periods of said pulses ofresultant voltage.

4. The method of treating a dry plate element of non-symmetricalconductivity which includes providing an alternating voltage having asine wave-form, providing a biasing direct voltage to produce adistorted alternating voltage from said first named voltage, impressingsaid distorted voltage upon said element in such manner that the maximuminstantaneous values of the voltage pulses of said distorted voltage aresubstantially greater in the blocking direction of said element than inthe current conducting direction thereof, and impressing a directvoltage upon said distorted voltage to determine the time periods ofsaid pulses of said distorted voltage.

HEINZ WITTKE.

